This invention relates to a ferromagnetic double quantum well tunneling magnetoresistance device for use in devices such as magnetic sensors, magnetic sensor heads, magnetoresistance effect devices and nonvolatile storages and so forth, which device utilizes a tunneling magnetoresistance effect that is brought about across two quantum wells by controlling directions of magnetizations caused in the two quantum wells.
A conventional tunneling magnetoresistance device makes use of rather a simple, three layer structure such as of a ferromagnetic metallic layer, an insulator layer and a ferromagnetic metallic layer. In such a device the ferromagnetic metallic layers in many cases are relatively thick with a thickness as large as several tens nanometers. What is utilized in the device is not a two-dimensional electron system by quantum confinement but a tunnel phenomenon in a three-dimensional electron system.
For this reason the need arises to use a ferromagnetic metal whose spin polarization is large enough to gain a large tunneling magnetoresistance, but spin polarization of ordinary ferromagnetic metals are not so much large and they are limited to at most 40 to 45%. While by using a ferromagnetic material called xe2x80x9chalf metalxe2x80x9d whose spin polarization is 100% it is expected that ideally an infinitely great value of tunneling magnetoresistance ratio, i.e., a change in electric resistance by application of a magnetic field divided by electric resistance when magnetization is parallel, is obtainable, this option has left problems to be resolved in respect of material engineering and fabrication techniques.
On the other hand, ordinary devices of the three-layer structure of a ferromagnetic metallic layer, an insulator layer and a ferromagnetic metallic layer has presented the problem that the tunneling magnetoresistance decreases in magnitude with an increase in the bias voltage, which must be resolved.
Accordingly, with these problems borne in mind, it is a first object of the present invention to provide a ferromagnetic double quantum well tunneling magnetoresistance device that derives an infinitely great value of tunneling magnetoresistance ratio with a desired bias voltage, by utilizing a two-dimensional electron (or positive hole) system.
A second object of this invention is to provide a sensitive magnetic sensor that is capable of detecting magnetism with an enhanced sensitivity.
A third object of this invention is to provide a nonvolatile storage that is both readable and writable.
In order to achieve the first object mentioned above, there is provided in accordance with the present invention a construction comprising a first and a second quantum well layer of ferromagnetic material each of which is sandwiched between barrier layers of non-magnetic material, in which a change in direction of magnetizations in the said first and second quantum layers causes carriers to tunnel between them, thereby producing a change in magnetoresistance across them.
Further, in addition to the construction mentioned above, the said first and second quantum well layers in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention have a difference in coercive force.
Also, the said first and second quantum well layers in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention have a thickness less than a de Broglie wavelength of the carriers.
Also, the said first and second quantum well layers in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention has a two-dimensional electron or positive hole condition established therein for subjecting the carriers to a quantum confinement.
Further, each of the said first and second quantum well layers and the said barrier layer adjacent thereto in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention has between them a hetero interface that is atomically flat and abrupt.
Also, the said barrier layers in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention are formed each to have a thickness that enables the carrier to tunnel therethrough.
Further, the said first quantum well layer in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention is made of either a ferromagnetic metal or a semiconductor exhibiting a ferromagnetism.
Also, the said second quantum well layer in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention is made of either a ferromagnetic metal or a semiconductor exhibiting a ferromagnetism.
Further, the said barrier layer in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention is made of either a nonmagnetic semiconductor or nonmagnetic insulator.
Also, the said first and second quantum well layers in the ferromagnetic double quantum well tunneling magnetoresistance device according to the present invention has a quantum level of a given energy that, and the carriers have a given tunneling probability that, are set up by regulating the thickness of each of the said first and second quantum well layers and the individual thickness and barrier height of the said barrier layers.
Further, the device according to the present invention is operable with a bias voltage that is set up at a desired magnitude by changing the thickness of each of the said first and second quantum well layers.
In a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, the carriers confined in each quantum well form quantum levels and make up a two-dimensional electron (positive hole) system. Because the quantum well is ferromagnetic, the quantum levels are spin-split into a spin level that points upwards and a spin level that points downwards.
Also, in a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, if the two (first and second) quantum well layers of ferromagnetic material are substantially equal in thickness, when they are magnetized parallel to each other in their pointing direction of magnetizations, their respective, two neighboring quantum levels that are equal in energy are equal in spin direction. Then, tunneling of the carriers between the two quantum wells easily takes place.
At Also, in a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, when to the contrary the two quantum well layers are magnetized antiparallel to each other or opposite in their pointing directions of magnetizations, their respective, two neighboring quantum levels that are equal in energy are opposite to each other in spin direction. Then, tunneling of the carriers between the two quantum wells is inhibited.
Also, in a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, a large difference in the tunneling probability between the two quantum wells between the state in which they are magnetized parallel as above and the state in which they are magnetized antiparallel as above, brings about a large change (difference) in electric resistance between the electrodes across them when electric current is being passed in the direction perpendicular to their interface.
Further, in a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, having the two (first and second) quantum well layers of ferromagnetic material had a difference in coercive force permits readily changing their states of magnetization controllably by manners in which an external magnetic field is applied thereto. This in turn permits establishing two distinct states, i.e., one in which the magnetizations are parallel and the electric resistance is low and the other in which the magnetizations are antiparallel and the electric resistance is extremely high, ideally infinitely great.
Accordingly, a ferromagnetic double quantum well tunneling magnetoresistance device of the present invention that permits external magnetic fields to steadily establish one state in which the two quantum wells are magnetized parallel in their pointing direction and the other state in which they are magnetized antiparallel, makes it possible to achieve a magnetoresistance effect much greater than the conventional device without recourse to a double quantum well.
Finally, a ferromagnetic double quantum well tunneling magnetoresistance device of such a construction as described above, allows the amount of energy of a quantum level and the probability of tunneling of carriers to be set up by controlling on an atomic level the layer thickness of a quantum well and the thickness and barrier height of barrier layers. A suitable bias voltage as desired can thereby be set up.
In order to achieve the second object mentioned above, there is also provided in accordance with the present invention a sensitive magnetic sensor that is constructed to incorporate a ferromagnetic double quantum well tunneling magnetoresistance device of the present invention, the magnetic sensor utilizing the nature of the ferromagnetic double quantum well tunneling magnetoresistance device of the invention that a change in direction of magnetizations in a said first and a said second quantum layer is created according to an external magnetic field which the device is placed under, and changes a carriers"" tunneling current between them, thereby producing a change in magnetoresistance across them.
A magnetic sensor so constructed as above according to the present invention, provided the ability to establish two distinct states in the first and second double quantum well layers, i.e., one state in which the magnetizations are parallel and the electric resistance is low and the other in which the magnetizations are antiparallel and the electric resistance is extremely high, ideally infinitely great, is capable sensing magnetism with an increased sensitivity and reliability.
In order to achieve the third object mentioned above, there is also provided in accordance with the present invention a three terminal ferromagnetic double quantum well tunneling magnetoresistance nonvolatile storage device, which is constructed to incorporate a ferromagnetic double quantum well tunneling magnetoresistance device of the present invention and further to include an electrode (third electrode) provided for each of a said first and a said second quantum well layer of the ferromagnetic double quantum well tunneling magnetoresistance device of the invention.
In a three terminal ferromagnetic double quantum well tunneling magnetoresistance nonvolatile storage device so constructed as above according to the present invention, the third electrodes can have voltages applied thereacross whose difference in voltage is capable of controllably establishing and de-establishing a resonant tunneling state between the first and second quantum well layers, thereby to selectively enable and disable reading a state of magnetizations of the first and second quantum well layers.
Also, in order to achieve the third object mentioned above, there is further provided in accordance with the present invention a readable and writable nonvolatile storage that comprises storage cells each of which comprises a said ferromagnetic double quantum well tunneling magnetoresistance device and a semiconductor switching device.
A readable and writable nonvolatile storage so constructed as above according to the present invention permits reading and writing at an increased speed.
Also, in order to achieve the third object mentioned above, there is further provided in accordance with the present invention a readable and writable nonvolatile storage that comprises storage cells each of which comprises a three terminal ferromagnetic double quantum well tunneling magnetoresistance nonvolatile storage device.
A readable and writable nonvolatile storage so constructed as above according to the present invention not only enables reading and writing at an increased speed, but, by eliminating the need to use a semiconductor switching device, permits storage cells to be integrated to a high degree.